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      MySQL学习笔记_day7
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    <h2 id="7-DDL语言"><a href="#7-DDL语言" class="headerlink" title="7 DDL语言"></a>7 DDL语言</h2><ul>
<li>Data Define Language数据定义语言 ：用于对数据库和表的管理和操作</li>
</ul>
<h3 id="7-1-库和表的管理"><a href="#7-1-库和表的管理" class="headerlink" title="7.1 库和表的管理"></a>7.1 库和表的管理</h3><blockquote>
<p>1.库的管理<br>创建、修改、删除<br>2、表弟管理<br>创建、修改、删除</p>
<p>创建：create<br>修改：alter<br>删除：drop</p>
</blockquote>
<h4 id="7-1-1-库的管理"><a href="#7-1-1-库的管理" class="headerlink" title="7.1.1 库的管理"></a>7.1.1 库的管理</h4><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token comment">#---------------------------库的管理------------------------√</span>

<span class="token comment">#一、创建数据库</span>
<span class="token keyword">CREATE</span> <span class="token keyword">DATABASE</span> stuDB<span class="token punctuation">;</span>
<span class="token keyword">CREATE</span> <span class="token keyword">DATABASE</span> <span class="token keyword">IF</span> <span class="token operator">NOT</span> <span class="token keyword">EXISTS</span> stuDB<span class="token punctuation">;</span>

<span class="token comment">#二、库的修改</span>
<span class="token keyword">RENAME</span> <span class="token keyword">DATABASE</span> books <span class="token keyword">TO</span> 新库名<span class="token punctuation">;</span>
<span class="token keyword">ALTER</span> <span class="token keyword">DATABASE</span> books <span class="token keyword">CHARACTER</span> <span class="token keyword">SET</span> gbk<span class="token punctuation">;</span> <span class="token comment">#更改库的字符集</span>

<span class="token comment">#三、删除数据库</span>
<span class="token keyword">DROP</span> <span class="token keyword">DATABASE</span> stuDB<span class="token punctuation">;</span>
<span class="token keyword">DROP</span> <span class="token keyword">DATABASE</span> <span class="token keyword">IF</span> <span class="token keyword">EXISTS</span> stuDB<span class="token punctuation">;</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h4 id="7-1-2-表的管理"><a href="#7-1-2-表的管理" class="headerlink" title="7.1.2 表的管理"></a>7.1.2 表的管理</h4><p>一、创建表 ★</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql">语法：
<span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> <span class="token punctuation">[</span><span class="token keyword">IF</span> <span class="token operator">NOT</span> <span class="token keyword">EXISTS</span><span class="token punctuation">]</span> 表名<span class="token punctuation">(</span>
	字段名  字段类型  【字段约束】<span class="token punctuation">,</span>
	字段名  字段类型  【字段约束】<span class="token punctuation">,</span>
	字段名  字段类型  【字段约束】<span class="token punctuation">,</span>
	字段名  字段类型  【字段约束】<span class="token punctuation">,</span>
	字段名  字段类型  【字段约束】
<span class="token punctuation">)</span><span class="token punctuation">;</span>

案例：没有添加约束
<span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> <span class="token keyword">IF</span> <span class="token operator">NOT</span> <span class="token keyword">EXISTS</span> stuinfo<span class="token punctuation">(</span>
	stuid <span class="token keyword">INT</span> <span class="token punctuation">,</span>
	stuname <span class="token keyword">VARCHAR</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
	stugender <span class="token keyword">CHAR</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
	email <span class="token keyword">VARCHAR</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
	borndate <span class="token keyword">DATETIME</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>

案例：添加约束（通用写法）
<span class="token keyword">DROP</span> <span class="token keyword">TABLE</span> <span class="token keyword">IF</span> <span class="token keyword">EXISTS</span> stuinfo<span class="token punctuation">;</span>
<span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> <span class="token keyword">IF</span> <span class="token operator">NOT</span> <span class="token keyword">EXISTS</span> stuinfo<span class="token punctuation">(</span>
	stuid <span class="token keyword">INT</span> <span class="token keyword">PRIMARY</span> <span class="token keyword">KEY</span><span class="token punctuation">,</span><span class="token comment">#添加了主键约束</span>
	stuname <span class="token keyword">VARCHAR</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span> <span class="token keyword">UNIQUE</span> <span class="token operator">NOT</span> <span class="token boolean">NULL</span><span class="token punctuation">,</span><span class="token comment">#添加了唯一约束+非空</span>
	stugender <span class="token keyword">CHAR</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token keyword">DEFAULT</span> <span class="token string">'男'</span><span class="token punctuation">,</span><span class="token comment">#添加了默认约束</span>
	email <span class="token keyword">VARCHAR</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span> <span class="token operator">NOT</span> <span class="token boolean">NULL</span><span class="token punctuation">,</span>
	age <span class="token keyword">INT</span> <span class="token keyword">CHECK</span><span class="token punctuation">(</span> age <span class="token operator">BETWEEN</span> <span class="token number">0</span> <span class="token operator">AND</span> <span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">,</span><span class="token comment">#添加了检查约束，mysql不支持</span>
	majorid <span class="token keyword">INT</span><span class="token punctuation">,</span>
	<span class="token keyword">CONSTRAINT</span> fk_stuinfo_major <span class="token keyword">FOREIGN</span> <span class="token keyword">KEY</span> <span class="token punctuation">(</span>majorid<span class="token punctuation">)</span> <span class="token keyword">REFERENCES</span> major<span class="token punctuation">(</span>id<span class="token punctuation">)</span><span class="token comment">#添加了外键约束</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>二、修改表[了解]</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql">语法：<span class="token keyword">ALTER</span> <span class="token keyword">TABLE</span> 表名 <span class="token keyword">ADD</span><span class="token operator">|</span><span class="token keyword">MODIFY</span><span class="token operator">|</span>CHANGE<span class="token operator">|</span><span class="token keyword">DROP</span> <span class="token keyword">COLUMN</span> 字段名 字段类型 【字段约束】<span class="token punctuation">;</span>

<span class="token comment">#1.修改表名</span>
<span class="token keyword">ALTER</span> <span class="token keyword">TABLE</span> stuinfo <span class="token keyword">RENAME</span> <span class="token keyword">TO</span> students<span class="token punctuation">;</span>

<span class="token comment">#2.添加字段</span>
<span class="token keyword">ALTER</span> <span class="token keyword">TABLE</span> students <span class="token keyword">ADD</span> <span class="token keyword">COLUMN</span> borndate <span class="token keyword">TIMESTAMP</span> <span class="token operator">NOT</span> <span class="token boolean">NULL</span><span class="token punctuation">;</span>
<span class="token keyword">DESC</span> students<span class="token punctuation">;</span>

<span class="token comment">#3.修改字段名</span>
<span class="token keyword">ALTER</span> <span class="token keyword">TABLE</span> students CHANGE <span class="token keyword">COLUMN</span> borndate birthday <span class="token keyword">DATETIME</span> <span class="token boolean">NULL</span><span class="token punctuation">;</span>

<span class="token comment">#4.修改字段类型</span>
<span class="token keyword">ALTER</span> <span class="token keyword">TABLE</span>  students <span class="token keyword">MODIFY</span> <span class="token keyword">COLUMN</span> birthday <span class="token keyword">TIMESTAMP</span> <span class="token punctuation">;</span>

<span class="token comment">#5.删除字段</span>
<span class="token keyword">ALTER</span> <span class="token keyword">TABLE</span> students <span class="token keyword">DROP</span> <span class="token keyword">COLUMN</span> birthday<span class="token punctuation">;</span>
<span class="token keyword">DESC</span> students<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p>三、删除表 √</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">DROP</span> <span class="token keyword">TABLE</span> <span class="token keyword">IF</span> <span class="token keyword">EXISTS</span> students<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<p>四、复制表√</p>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token comment">#仅仅复制表的结构</span>
<span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> newTable2 <span class="token operator">LIKE</span> major<span class="token punctuation">;</span>

<span class="token comment">#复制表的结构+数据</span>
<span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> newTable3 <span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> girls<span class="token punctuation">.</span><span class="token punctuation">`</span>beauty<span class="token punctuation">`</span><span class="token punctuation">;</span>

<span class="token comment">#案例：复制employees表中的last_name,department_id,salary字段到新表 emp表，但不复制数据</span>
<span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> emp 
<span class="token keyword">SELECT</span> last_name<span class="token punctuation">,</span>department_id<span class="token punctuation">,</span>salary 
<span class="token keyword">FROM</span> myemployees<span class="token punctuation">.</span><span class="token punctuation">`</span>employees<span class="token punctuation">`</span>
<span class="token keyword">WHERE</span> <span class="token number">1</span><span class="token operator">=</span><span class="token number">2</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h3 id="7-2-数据类型"><a href="#7-2-数据类型" class="headerlink" title="7.2 数据类型"></a>7.2 数据类型</h3><pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token number">1</span>、整型
<span class="token keyword">TINYINT</span> <span class="token keyword">SMALLINT</span>  <span class="token keyword">INT</span>  <span class="token keyword">BIGINT</span> 

<span class="token number">2</span>、浮点型
<span class="token keyword">FLOAT</span><span class="token punctuation">(</span>m<span class="token punctuation">,</span>n<span class="token punctuation">)</span>
<span class="token keyword">DOUBLE</span><span class="token punctuation">(</span>m<span class="token punctuation">,</span>n<span class="token punctuation">)</span> 
<span class="token keyword">DECIMAL</span><span class="token punctuation">(</span>m<span class="token punctuation">,</span>n<span class="token punctuation">)</span>
m:整数部位长度<span class="token operator">+</span>小数部位长度
n:小数部位长度
m和n可选

<span class="token number">3</span>、字符型
<span class="token keyword">CHAR</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span>:n可选
<span class="token keyword">VARCHAR</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span>：n必选 
<span class="token keyword">TEXT</span> 
<span class="token keyword">ENUM</span>
<span class="token keyword">SET</span>
n表示最多字符个数

<span class="token number">4</span>、日期型
<span class="token keyword">DATE</span> <span class="token keyword">TIME</span>  <span class="token keyword">DATETIME</span> <span class="token keyword">TIMESTAMP</span>

<span class="token number">5</span>、二进制型
<span class="token keyword">BLOB</span> 存储图片数据<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<h3 id="7-3-常见约束"><a href="#7-3-常见约束" class="headerlink" title="7.3 常见约束"></a>7.3 常见约束</h3><ul>
<li>说明：用于限制表中字段的数据的，从而进一步保证数据表的数据是一致的、准确的、可靠的！</li>
</ul>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token operator">NOT</span> <span class="token boolean">NULL</span> 非空：用于限制该字段为必填项<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">DEFAULT</span> 默认：用于限制该字段没有显式插入值，则直接显式默认值<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>
<pre class="line-numbers language-none"><code class="language-none">PRIMARY KEY 主键：用于限制该字段值不能重复，设置为主键列的字段默认不能为空
	一个表只能有一个主键，当然可以是组合主键<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>
<pre class="line-numbers language-none"><code class="language-none">UNIQUE 唯一：用于限制该字段值不能重复
		字段是否可以为空   一个表可以有几个
		
	主键	   ×				1个
	唯一     √				n个<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<pre class="line-numbers language-none"><code class="language-none">CHECK检查：用于限制该字段值必须满足指定条件
	CHECK(age BETWEEN 1 AND 100)<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>
<pre class="line-numbers language-sql" data-language="sql"><code class="language-sql"><span class="token keyword">FOREIGN</span> <span class="token keyword">KEY</span> 外键:用于限制两个表的关系<span class="token punctuation">,</span>要求外键列的值必须来自于主表的关联列
	要求：
	①主表的关联列和从表的关联列的类型必须一致，意思一样，名称无要求
	②主表的关联列要求必须是主键
	③插入数据时，先插入主表，再插入从表
	④删除数据时，先删除从表，再删除主表<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

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